The seaquake forces acting a floating body were calculated numerically byusing two-dimensional and three-dimensional BIEM,and two-dimensional time domain solver. The effects of marine sediment were also investigated. The results may be summarized as follows :1) There are resonances corresponding to the eigen value of the boundary problem on the seaquake forces.2) The inteval of resonat frequencies become smaller with the increase of the index of refraction between sea-water and marine sediment.3) The interval of resonat frequencies become smaller with the increase of the marine sediment's thickness.4) The floating structure may be constructed in the ocean area, where the marine sediment' thickness is thin, from an aseismatic point of view.5) The magnetude of seaquake forces at resonance in three-dimension are not larger than those in two-dimension because of wave spreading.6) The structure safty must be paied attention, when the earthquake occurs beneath the structure.7) The instantaneous large seaquake force may act on the structure in transient condition.8) The elastic body may vibrate largely in some special conditions by the seaquake.